1. Field of the Invention
The invention relates to the field of peak-limiting circuits particular for audio signals.
2. Prior Art
Numerous peak-limiting circuits, methods and systems are known for peak-limiting audio signals. The peak levels of such signals must be controlled in systems characterized by a sudden overload point which may not be exceeded for satisfactory operation. Among such systems are AM broadcasting, FM broadcasting, optical film recording by the "variable area" method, carrier telephony, and disc recording. The present invention is especially useful for systems characterized by a steep high frequency rolloff in a receiver or other device and where high frequency pre-emphasis is used to compensate for this rolloff, such as in AM broadcasting.
One system for controlling peak levels in AM broadcasting employs a variable gain amplifier, or like element, in the audio path. This amplifier is controlled by a level detector which typically includes some form of integration at its output. For audio signals, the attack time for this system is in the range of ten microseconds with a recovery time of a few hundred milliseconds. Unnatural volume reductions often occur with this sytem because microsecond-wide peaks can cause gain reductions for hundreds of milliseconds.
Another prior art peak control means employs clipping; diode clippers are used to instantaneously clip the waveform. The amount of such clipping is usually quite small (2 to 5 db) to prevent noticeable non-linear distortion.
A variable gain amplifier in combination with a clipper is also used for controlling peak levels of audio signals for AM broadcasting. In this system, the attack time of the variable gain amplifier loop is slower than the attack time for such amplifiers when used without a clipper. These slower attack times can result in audible distortion when clipping does occur. The attack time in these systems must be fast enough to prevent noticeable clipping distortion for the "worse case" program material, thus this system cannot be optimized for maximum loudness.
The prior art systems all lack information concerning the amount of distortion perceived by a listener. The action of these systems is conservative in order not to introduce noticeable distortion. For this reason loudness is not maximized for many types of program material.
As will be seen, the presently described apparatus and method permits maximum possible loudness from a peak-level limited channel at all times by clipping the signal as much as possible without adding noticeable distortion. This method is particularly effective with systems employing large amounts of high frequency pre-emphasis. The level of noticeable audio distortion is estimated based on known psychoacoustic masking and this estimate is used in a feedback loop to control the signal level applied to a clipper.